Encoders are used to measure the angular position of a rotating element, or the relative displacement of sliding elements. They are typically used in control systems, often referred to as servo systems, where a motion controller is used to make a moving element follow a precise desired path. For that purpose, encoder devices include an electronic interface which allows their connection to a motion controller.
Encoders may be of two types, rotary and linear. Rotary encoders are designed to measure the angular position of a rotary element, like the shaft of a motor or any rotating device. Linear encoders are designed to measure the relative movement of two sliding elements, for example a sliding carriage mounted on a linear bearing relative to a static base.
In a common application, a rotary encoder is mounted on an electrical motor shaft at the rear end, and provides position information about the shaft rotation angle to the electric motor controller. The motor controller will then output an appropriate current to the motor in order to make it rotate toward the desired position.
In another common application, a linear encoder is mounted on the moving element of a linear motor, and is connected to the linear motor motion controller.
Throughout this patent application, the term “encoder device” shall refer to both a rotary encoder and to a linear encoder.
In automatic machinery, it is often required that moving elements will follow a path with very high precision and at high speeds. To achieve this, the encoder device should have a high precision, and should be able to transfer position information at a high rate. As an example, commercially available rotary encoders can provide precision better than 0.01 degrees when the rate of transfer of the rotation angle data to a motion controller is typically between 8,000 to 30,000 data transfers per second.
Another quality required from an encoder device is its resolution. The resolution represents the number of positions that the encoder device is able to measure in one revolution or in one unit of length. The resolution is usually higher than the precision, meaning that the encoder device is able to provide position data having more significant digits than required for the precision, even if the position value output differs from the actual position by some error, this error being inferior to that defined by the precision characteristic of the encoder. High resolution allows motion controllers, also called servo controllers, to achieve a tight and smooth control of the moving elements.
Encoder devices may be absolute or incremental. An absolute encoder device is able to measure the angular or linear position relative to a fixed reference position, while an incremental encoder device is capable of measuring the angular or linear displacement from the start of its operation. Thus, when an incremental encoder device is used in the automatic machinery, it is common to execute, at each start of operation of the machine, a search for a reference position. This search is done at a slow speed in a given direction, until a limit switch, or other device placed at the reference position, is activated. This search procedure adds complexity to the system, and delays the first operation of the machine. In spite of this drawback, incremental encoders are commonly used, due to their simplicity and their low cost. In many cases, a machine builder would have preferred to use an absolute encoder, but makes use of an incremental encoder due to the higher cost of presently available absolute encoders.
The absolute resolution of an absolute encoder device is limited by the number of sensors. An encoder using a number n of sensors can have a maximum absolute resolution of 2n. For example, an 8 sensor rotary encoder cannot provide absolute resolution of 256. In order to obtain a higher resolution, absolute encoder devices are usually combined with a high resolution incremental encoder device in order to provide a high absolute resolution encoder device. This results in higher complexity, size and cost of the device.
It is thus desirable to provide an absolute encoder device, which is of simple fabrication and still provides high precision and resolution at a lower cost.
In Villaret (USA Published Patent Application 2010/0140463), there is described an absolute encoder device of simple construction, that can provide absolute position information. The device makes use of a number of sensors, equally distributed on a circumference of a static part. A rotating disc, having sections of alternating properties on an annular track, is placed so that the sensors can sense the property of the section of track in proximity. During disc rotation, different sections of the rotating disc come into proximity to each sensor. Each sensor electrical signal is digitized to provide a bit value 1 or 0. Bit values of all sensors are then combined in a digital word to create a unique code value for each rotating disc angular range position. An advantage of Villaret is the simplicity of the device. Since sensors are equally distributed on a circular line, distance between sensors is relatively large and commercially available sensors of normal size can be used.
In the herein patent application, the absolute resolution or N is the number of code values generated while rotating the encoder disc by a full turn. The “Sectors” are defined as being angular portions of equal size of an encoder rotating disc circular track. The number of sectors is equal to N, the encoder absolute resolution. Each sector of the said track is made of material having a first or a second property, according to a predefined pattern.
The encoders include sensors that are placed in proximity of the disc circular track and are sensible to the property of the closest sector of the track. For example, many optical encoders have a circular track including transparent and opaque sectors. A light emitter is placed on one side of the rotating disc, and the light sensors are placed on the other side, so that light passing through the transparent sectors is sensed by the light sensors. Whenever light is sensed by a sensor, this sensor outputs a signal represented by the digital value 1, indicating a transparent sector; and, whenever there is no light sensed, the sensor will output a signal represented by the digital value 0.
For purposes of understanding the prior art, a prior art encoder embodiment with a relative low resolution is described. For these descriptions, particular values of the number of sectors N and the number of sensors S are used. It must be understood that other values of N and S can be used.
Ohno (U.S. Pat. No. 5,068,529) describes an absolute encoder using a first patterned track to measuring incremental position, and a second patterned track to measure absolute position.
In FIG. 1 herein, there is shown an encoder built according to a first prior art embodiment, providing an absolute position resolution of 32. A rotating disc 101 is mounted on a rotating shaft 102, and includes a circular track indicated by the dashed line 107. The circular track 107 is composed of a number N=32 of sectors, for example 103, all of equal size. Each sector has a first or second property according to a defined pattern. For example, a sector of first property may be transparent, and a sector of second property may be opaque.
A number S=5 of sensors, 105a-105e, are fixed and disposed on a circular path in proximity to the rotating disc circular track, so that sensors 105a-105e sense the property of the closest sector and output S digital signals b0-b4, representing values 0 or 1 according to the closest sector property. These S digital signals are then combined in one digital word 106, whose value is characteristic of the angular position of the rotating disc 101.
According to this patent application, in order to indicate the angular position of the rotating disc, the term “sector position” is used. The rotating disc is in a sector position p when the rotating disc is in an angular position such that sector number p of the rotating disc circular track 101 is the closest to a reference sensor, for example sensor 105a. There are thus N possible sector positions for the rotating disc.
The pattern of properties of the sectors is designed so that each value of the word 106 obtained at a given sector position is never obtained at a different sector position of the rotating disc. Nagase (U.S. Pat. No. 5,117,105) describes a method to design such a pattern.
A first drawback of this prior art embodiment is the requirement that the sensors used to measure the absolute position must be placed at angular distances equal to a sector's angular size. This requires that the sensors should be of a smaller size than a sector. For example, if the rotating disc diameter is 30 mm and the absolute resolution required is 256, each sensor should be smaller than 0.36 mm. In that case commercially available sensors cannot be used, and custom sensors, integrated on one chip, must be used.
Typically, this can be done using optical sensing devices, where several sensors are implemented on one semiconductor device. This implementation is not practical, due to the high cost and the lack of modularity. For each encoder size, a different integrated device should be designed. If magnetic sensors are considered, the design of small size integrated device is even more complex and expensive.
A second drawback of this prior art embodiment is the fact that resolution is limited. Due to practical considerations on the size of the sensors and the size of the sectors, the number of sectors is limited, and thus also the encoder resolution. Whenever high resolution is required, the absolute encoder is typically combined with an incremental encoder, as described for example in Imai (U.S. Pat. No. 5,252,825). This results in increased complexity and cost of the encoder.
Villaret (US Published Patent Application US2010/0140463) provides an improvement that eliminates the first above-mentioned drawback. Referring to FIG. 2 herein, according to Villaret, a rotating disc 201, fixed to a rotating shaft 203, includes a circular track divided into a number N=20 of sectors, like 202a and 202b. Each sector has a first or second property. In FIG. 2, black sectors 202a represents a first property, and white sectors 202b represents a second property. A number S=5 of sensors 205a-205e are positioned on a fixed element, in proximity above the circular track and output digital signals (bits) B0-B4. The S=5 bits are combined in a digital word 206 that selects one among N=20 digital values. Since the S sensors are equally distributed on the circumference, they can be of usual size of commercially available devices, thus simplifying the encoder design, and reducing its costs.
Villaret, while eliminating the first drawback mentioned above, however still suffers from a limited resolution, due to practical limitations in the size of the sectors. Whenever sectors become smaller, it is necessary to place the sensor at a very short distance from the rotating disc circular track, so that it will be sensible to the closest sector only. If a too large a number N of sectors were used, then this distance would become smaller than the mechanical tolerances of the encoder parts, thus making it inoperable.
It is thus desirable to provide an absolute encoder device of simpler construction, smaller size and lower cost. It would be desirable to provide an absolute encoder having high absolute resolution, wherein commercially available sensors can be used and wherein no additional patterned track is required.